Non-volatile memory devices are beneficial in certain applications where data must be retained when power is disconnected. Applications include general memory cards, consumer electronics (e.g., digital camera memory), automotive (e.g., electronic odometers), and industrial applications (e.g., electronic valve parameter storage). The non-volatile memories may use phase-change memory materials, i.e., materials that can be switched between a generally amorphous and a generally crystalline state, for electronic memory applications. The memory of such devices typically comprises an array of memory elements, each element defining a discrete memory location and having a volume of phase-change memory material associated with it. The structure of each memory element typically comprises a phase-change material, one or more electrodes, and one or more insulators.
One type of memory element originally developed by Energy Conversion Devices, Inc. utilizes a phase-change material that can be, in one application, switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. These different structural states have different values of resistivity, and therefore each state can be determined by electrical sensing. Typical phase-change materials suitable for memory application include those incorporating one or more chalcogen or pnictogen elements. Unlike certain known devices, these electrical memory devices typically do not use field-effect transistor devices as the memory storage element. Rather, they comprise, in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little area is required to store a bit of information, thereby providing for inherently high-density memory chips.
Ovonic unified or phase-change memories are an emerging type of electrically-alterable non-volatile semiconductor memories. These memories exploit the properties of materials (phase-change materials) that can be reversibly switched between two or more structural states that vary in the relative proportions of amorphous and crystalline phase regions when subjected to heat or other forms of energy. The term “amorphous” refers to a condition which is relatively structurally less ordered or more disordered than a single crystal and has a detectable characteristic, such as high electrical resistivity. The term “crystalline” as used herein refers to a condition which is relatively structurally more ordered than amorphous and has at least one detectably different characteristic, such as a lower electrical resistivity.
The distinct structural states of a phase-change material exhibit different electrical characteristics, such as resistivity, that can be used to distinguish the different states. Memory or logic functionality is achieved by associating a different memory or logic value with each structural state. Programming occurs by providing the energy needed to stabilize the structural state of the phase-change material associated with the input memory or logic data.
Typically, a memory array includes a matrix of phase-change memory cells, arranged in rows and columns with associated word lines and bit lines, respectively. Each memory cell typically consists of a phase-change storage element connected in series to an access element, where each memory cell is connected between a particular word line and a particular bit line of the array. Each memory cell can be programmed to a particular memory state by selecting the word line and bit line associated with the memory cell and providing a suitable energy pulse across the memory cell. The energy pulse is typically a current pulse applied to the memory cell by supplying a voltage potential between the word line and bit line of the cell. The voltage potential activates the access element connected to the memory element, thereby enabling the flow of current through the memory element. Typical access elements include diodes and transistors. Reading of the memory state is accomplished by similarly selecting the word line and bit line of the memory cell and measuring the resistance (or a proxy therefore such as the voltage drop across the cell). In order to maintain the state of the memory cell during read, it is necessary to maintain the energy of the read signal at a level below that needed to transform the memory cell from its existing state to a different state.
Current embodiments of phase-change memory devices include a phase-change material in electrical communication with two or more metal contacts. An undesirable characteristic of many metal contacts is their tendency to react with or form alloys with the phase-change material. Alloying may occur at the interface between phase-change material and metal contact and may be facilitated by diffusion or electromigration of metal atoms during operation of the device. When reaction or alloying occurs, the memory device characteristics deteriorate. As a result, the operating life and performance of the device are compromised.
Therefore, a need has arisen to improve and maintain device performance over time. Moreover, it is desirable to reduce or substantially prevent reaction or alloying of metal contacts with the phase-change material of the memory device. It is also desirable to provide a contact having a high temperature resistance and high material stability.